1. Field of the Invention
This invention relates generally to electronic circuits and, in particular, to phase locked loop and delay locked loop circuits used in electronic circuits.
2. Description of the Related Art
Consumer and commercial electronics must meet FCC electromagnetic emissions standards. PLL circuits, like other electronic circuits, also generate electromagnetic emissions that must meet FCC standards. In some existing systems, this is accomplished by adding expensive and heavy shielding. In other existing systems, it is accomplished by implementing a digital spread spectrum technique where a clock signal has its frequency modulated in a controlled manner around a center frequency. As noted above, the shielding technique is expensive and physically heavy. On the other hand, the digital spread spectrum technique is rigid as it involves setting counters (or dividers) to one set of predetermined values, and changing the counter settings to a second set of predetermined values to achieve a predetermined frequency modulation.
Another existing system, uses analog spread spectrum modulation of currents. There are a number of disadvantages of using current modulation. One, it is difficult to generate a triangular waveform with current modulation. Second, the output of the current modulation is a current which is not the most desirable parameter with which to control a voltage controlled oscillator.
The present invention addresses this and other disadvantages of existing current reference circuits.
The present invention uses an analog approach to modulate the control voltage in a phase locked loop. One aspect of the analog approach of the present invention, unlike the existing digital approach, provides a more controlled modulation without having to resort to resetting counters to specific predetermined values. Thus, the analog approach de-couples the counters from the modulation, providing a more flexible modulation scheme. One aspect of the spread spectrum modulator of the present invention allows for easily changing the spread mode (i.e., the type of spread) and the percentage of spread provided by the spread spectrum modulator. Another aspect of the spread spectrum modulator of the present invention provides for additional filtering that may be included to reduce high frequency spurs. In another aspect, the spread spectrum modulator of the present invention provides spread spectrum modulation independent of the process, supply voltage, and temperature.
The present invention encompasses a PLL circuit. In one embodiment, the PLL circuit of the present invention includes: a signal generator; and a spread spectrum modulator coupled to the signal generator, where the spread spectrum modulator receives a control voltage as an input and provides a spread spectrum control voltage to the signal generator in response to the control voltage. In one embodiment, the spread spectrum modulator includes at least one selector, where the at least one selector selects a plurality of voltage levels that correspond to a spread mode and percentage of spread for the spread spectrum modulator.
In one embodiment, the phase locked loop circuit includes a spread spectrum modulator, where the spread spectrum modulator comprises a voltage divider and a selector coupled to the voltage divider, where the selector selects a plurality of voltages that correspond to a spread rate and percentage of spread for the spread spectrum modulator. In one embodiment, the selector includes a plurality of multiplexers, where a first multiplexer of the plurality of multiplexers selects a high voltage, a second multiplexer of the plurality of multiplexers selects a low voltage, and a third multiplexer of the plurality of multiplexers selects a reference voltage.
The present invention is explained in more detail below with reference to the drawings.